III-V compound semiconductors, especially of gallium arsenide, find a wide variety of applications for industrial purposes on the basis of their high electron mobility, direct transion behaviour, transfered-electron efect and so on. Many kinds of electric devices such as high electron mobility transistors, light emitting diodes and Gunn diodes have been realized on various gallium arsenide substrates.
During the fabrication of the electric devices on the gallium arsenide substrates, ionized impurity atoms, typically silicon atoms, are implanted into the substrate with enough energy to penetrate beyond the surface portion in order to impart a conductivity type in selected regions. After the ion implantation, the substrate is annealed for activation of the introduced inpurity atoms in the substrate. The above steps are combined with other well-known techniques so as to complete a manufacturing process through which electric elements are fabricated on the semi-insulating substrate.
When the ion implanted substrate is annealed, the arsenic atoms tend to volatilize or out-diffuse from the substrate by the agency of heat applied to the substrate. This volatilization causes the deep energy level EL2 concentration in the surface region of the substrate to reduce with a resultant of reduction in resistivity.
Namely, the deep energy level EL2 is considered to take place due to antisite defects, in which arsenic atoms occupy some gallium sites. If the substrate is heated to a certain temperature, some of the antisite arsenic atoms occuping gallium sites are out-diffused. Therefore, the antisite defects are healed and, then, the EL2 concentration is lowered. The high resistivity results from compensation of acceptors by the EL2 deep donors so that the resistance value of the surface region is reduced under the reduction of the deep energy level El 2 concentration. This reduction in resistivity is not desirable for an isolation region electrically separating the adjacent electric elements fabricated on the substrate because of the leakage currents across the isolation region.
Attempts have been made to protect the isolation region against the volatilization arsenic atoms. One of the attempt is to apply a protection layer over the substrate prior to the heat treatment. The protection layer prevents the volatilization of the arsenic atoms to some degree, however, it can not perfectly prohibit such volatilization. Moreover, the protection layer behaves as an impurity source so that the electric elements fabricated on the substrate are subjected to influence of impurities from the impurity source, thereby giving rise to deterioration in the electric characteristics of the elements. Another attempt to prevent volatilization of the arsenic atoms is such that the polished surfaces of a pair of substrates are held in face-to-face contact with each other and, then, placed in a furnace for heat treatment. However, the face-to-face contact merely reduces the volatilization to a slight degree. Another method proposed to eliminate the volatilization of the arsenic atoms involves heat treatment carried out in an ambient formed with a group V element. However, in this conventional method the ambient is prepared by the pyrolysis of arsine, which is instantly lethal if a concentration of 250 ppm is inhaled, and exposure at lower levels poses a health hazard depending upon the lenght of exposure. Therefore this method is not desirable for operators in view of their health. Another attempt employs a flush annealing technique, using beams with high energy densities in order to rapidly anneal the ion implanted substrate. However, when a wafer with a large diameter is employed to enhance productivity, flush annealing encounters a problem in the uniformity of the heat treatment.
Therefore it is an object of the invention to provide an improved annealing method which activates the implanted impurity atoms without reduction in resistivity of the isolation region in the substrate.
It is also an object of the invention to provide an improved annealing method which is free from the drawbacks inherent in the conventional methods.